Characterization of ceramic coatings fabricated on zirconium alloy by plasma electrolytic oxidation in silicate electrolyte

Thick ceramic coatings were prepared on zirconium alloy by plasma electrolytic oxidation (PEO) in silicate electrolyte. Their microstructure and phase constituent were analyzed. The micromechanical properties of a thick coating are measured by nanoindentation method. The electrochemical corrosion behaviors of the coatings were evaluated by potentiodynamic polarization. The coating consists of the monoclinic ZrO2 and tetragonal ZrO2, but the tetragonal ZrO2 content is much lower than the monoclinic ZrO2 content. In the inner layer coating, the nanohardness and elastic modulus are much higher than that of the Zr substrate, and their maximum values reach 7.4 GPa and 170 GPa, respectively. However, the elastic modulus of the coating is lower than that of Zr substrate when the distance is far from the coating/Zr interface. After PEO treatment, the corrosion potential of zirconium alloy moves to a more positive potential, and the corrosion current density reduces several orders of magnitude. However, the corrosion current density increases again while the coating is very thick. PEO is an effective method to improve corrosion resistance of zirconium, especially for pitting attack.